Pressure difference sensors are usually optimized to measure small pressure differences p1-p2 in the presence of large static pressures p1, p2. In such case, it is important to find the right balance between sensitivity and overload resistance. Thus, for example, |p1−p2|/p1<1% can hold for the measuring range of the pressure difference |p1−p2|. When in a process installation one of the pressures p1, p2 is absent, then the pressure difference sensor is loaded with 100s of times the measuring range. Pressure difference transducers are known, which withstand such overloads. A proven protection of the sensitive pressure difference measuring cells is to connect an overload membrane hydraulically in parallel with the pressure difference sensor, wherein the pressure difference measuring cell and the overload membrane are supplied with the two pressures p1, p2 via hydraulic paths, wherein the pressures are introduced through isolating diaphragms into the hydraulic paths. An overload membrane includes a sufficiently large hydraulic capacitance, in order in the case of a one-sided overload to accommodate the volume of a pressure transfer liquid in a hydraulic path sufficiently that the isolating diaphragm of this hydraulic path comes to rest on a diaphragm bed, so that a further rise of the pressure difference acting on the pressure difference sensor is reliably prevented. Examples of pressure difference transducers with overload membranes are disclosed in European Patent EP 1299 701 B1, and German Patents DE 10 2006 040 325 A1 and DE 10 2006 057 828 A1.
The use of overload membranes leads, however, necessarily to greater volume strokes of the pressure transfer liquid and therewith—in the case of equal performance—to greater separating membrane areas, which means greater device dimensions and higher costs. Moreover, the measuring mechanism dynamic range is negatively influenced by the overload membrane and the greater volume of the pressure transfer liquid.
There are, consequently, efforts known to implement the overload protection for the measuring membrane by membrane beds. In such case, the measuring membrane should in the case of exceeding a limit value for a one-sided positive pressure at least find sufficient support on the diaphragm bed that the bursting stress of the measuring membrane is not reached even in the case of an additional pressure rise.
For such purpose, especially suitable are aspherical membrane beds, which approximate the bend line of the measuring membrane at the limit value for the positive pressure.
U.S. Pat. No. 4,458,537 discloses a capacitive pressure difference sensor having an aspherical diaphragm bed of glass built in a structure of coaxial rings, wherein the heights of the rings form a contour, which corresponds to the bend line of the measuring membrane.
Offenlegungsschrift (laid open German application) DE 10 2009 046 229 A1 discloses a pressure sensor, respectively pressure difference sensor, having an aspherical diaphragm bed of glass, which is formed by thermal yielding.
U.S. Pat. No. 7,360,431 B2 discloses a pressure sensor, respectively pressure difference sensor, with an aspherical diaphragm bed, which is prepared in silicon by means of grayscale lithography.
Offenlegungsschrift (laid open German application) DE 10 2010 028 773 A1 discloses a pressure sensor, respectively pressure difference sensor, with an aspherical diaphragm bed, which is prepared in silicon by means of laser ablation, followed by an oxidation step and a terminal etching.
Although the mentioned membrane support concepts can actually protect the measuring membrane to a certain degree, nevertheless the static pressure introduced into the pressure difference sensor loads the joints between measuring membrane and platforms or regions bordering thereon, so that stress peaks can occur there, which lead to a destruction of the pressure difference sensor.
Apart from static overload pressures present on both sides, a one-sided supplying of the pressure difference measuring cell with a static overload pressure can also damage or destroy the measuring membrane, the platform or the joints between measuring membrane and platforms or regions bordering thereon, when the one-sided overload leads to deformations of the platform, whereby, for example, the support function for the membrane beds is degraded.
In order to counter this, Hein et al. (Transducers '97, pages 1477-1480, 1997) disclose an encapsulated capacitive pressure difference sensor, in the case of which the platforms are axially clamped between pressure connection pieces, wherein, in each case, between a platform and a pressure connection piece supplementally a sealing ring is clamped.
German patent DE 37 51 546 T2 likewise discloses a pressure difference sensor, which has a measuring membrane between two platforms, wherein the two platforms are axially clamped in an elastic clamping apparatus, in order to increase the bursting strength of the pressure difference sensor.
Common to the two above described arrangements is that, in the case of supplying the pressure difference sensor with static pressure, relative movements can occur between the platforms and the clamping apparatus. This can lead especially to hysteresis errors in the zero point and measuring range of a measurement signal of the pressure difference sensor dependent on the pressure difference.